Germanwings Airbus A320 Crash: Damaged Black Box Taken to Paris

SEYNE-LES-ALPES, France — Two Americans were believed to be on board the Germanwings plane that slammed into a remote Alpine mountainside, the airline said Wednesday, as investigators continued searching for clues on what caused the crash.

French officials said a black box recovered from the ill-fated plane — Flight 4U9525's cockpit voice recorder — was damaged but could still shed light on what prompted the Airbus A320 to descend rapidly and crashing into the Alps, killing all 150 people aboard. Some Germanwings crew members have refused to fly following the unexplained accident.

French Interior Minister Bernard Cazeneuve acknowledged that all possible explanations for the crash are being considered, but told RTL radio that terrorist action is not the most likely theory.

"We have not been able to contact all of the relatives yet," Germanwings Managing Director Thomas Winkelman told reporters, saying the information about nationalities was correct as of 11 a.m. (6 a.m. ET).

There were two passengers each from Australia, Argentina, Iran and Venezuela and one each from the U.K., Netherlands: Colombia. Mexico, Japan, Denmark and Israel, Winkelman added. U.S. officials were not immediately available for comment.

The victims included two babies, two opera singers, an Australian mother and her adult son vacationing together, and 16 German high schoolers returning with their teachers from an exchange trip to Spain.

France's air force says it scrambled a Mirage fighter jet to the area when the Germanwings flight lost radar contact on Tuesday but it arrived too late to help and didn't spot the wreckage.

In Seyne-les-Alpes, locals had offered to host bereaved families because of a shortage of rooms to rent, said the town's mayor, Francis Hermitte.

Claude Buzon, 67, who lives in a village near the crash site, said the doomed plane made a "low sound" unlike the noise normally made by passing jets. "Afterwards I heard no explosion, no impact, nothing," he said.

"It is inexplicable this could happen to a plane free of technical problems and with an experienced, Lufthansa-trained pilot," Lufthansa CEO Carsten Spohr told reporters in Frankfurt.

In Spain, flags flew at half-staff on government buildings and a minute of silence was held in government offices across the country. Parliament canceled its normal Wednesday session.

Germanwings workers at the company's Cologne headquarters and at several airports observed a one-minute silence to mark the tragedy at 10:53 a.m. local time, the moment the airline says the plane crashed.

Lufthansa — whose employees worldwide also observed a moment of silence — said the flight number 4U9525 had been retired.

The cockpit voice recorder tracks all conversations between the pilots as well as any noises in the cockpit. The flight data recorder, which has not yet been recovered, captures 25 hours' worth of information on the position and condition of almost every major part in a plane.

— NBC News' Nancy Ing and Carol Marquis and The Associated Press contributed to this report